Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions.
A light emitting device such as an LED is often combined with a wavelength converting material such as a phosphor. U.S. 2010/0289044 describes desirable properties of a red-emitting phosphor. In particular, “for >200 lm/W white down-conversion LEDs, red is the most critical spectral component since the spectral position and width of the red emission directly determines luminous efficacy and color rendition. Besides high efficiency and stability, a suitable Eu2+ doped host lattice for narrow emission red should fulfill at least part of the following requirements:
1. Strong, covalent activator-ligand interactions are needed to efficiently lower the net positive charge of the activator. A medium condensed nitride lattice with coordinating N[2] ligands is considered as most suitable.
2. The host should contain only one substitutional lattice site for the activator ion and no statistical site occupation within the host structure (as found for SiAlONes or CaSiAlN3:Eu) to avoid inhomogeneous broadening of the emission band. In case that more than one substitutional lattice is present in the host lattice, the substitutional lattice sites should differ significantly in chemical nature to avoid spectral overlap of emission bands.
3. The activator site should show a high symmetry to limit possible structural relaxation modes of the activator in the excited state. Preferably, the activator site is larger (Ba site) than Eu2+ to hinder excited state relaxation and thus minimize the Stokes shift.
U.S. 2010/0289044 further states Preferably, the red emitting Eu(II) phosphor should also show coordination numbers of the Eu(II) activator between 6 and 8 and an activator-ligand arrangement that leads to a strong splitting of the Eu(II) 5d levels required for red emission in combination with a small Stokes shift. The activator-ligand contact length should lie in the range 210-320 pm. In other words, a suitable red phosphor is characterized by a six fold to eightfold coordination of the red emitting activator by its ligands and activator-ligand contact lengths in the 210-320 pm range.”